Precoated sheet for use with a light sensitive reproduction coating



July 11, 1967 E. JAHODA ETAL 3,330,662

PRECOATED SHEET FOR USE WITH A LIGHT SENSITIVE REPRODUCTION COATING Filed Sept. 23, 1963 LIGHT SENS/T/VE 0/920 LAWER 8/955 SHEE T p/aMs/v TED PRE 6047 L 4YR INVENTORS [OW/9R0 JflHODfi 5 TER MUL 1. ER

44, M Mm 4 T' TORIVE Y5 United States Patent York Filed Sept. 23, 1963, Ser. No. 310,614 5 Uaims. (Cl. 96-75) This invention pertains to improved reproduction papers also known as blueprints, diazotypes or brownprints. In particular, the invention relates to improved pretreated or precoated base carrier sheets for such reproduction papers and to an improvement in the method of making the same.

Our improved precoat may be applied to the base sheet of reproduction papers at the prevailing speeds of sensitizing machines or at the much faster speeds of the coaters used in paper mills to coat book or magazine paper, which offer the advantage of a more economical application, and where the base sheet can be substantially upgraded thus permitting the use of a less costlier paper stock.

A further advantage of this invention is a more economical precoat cost and a precoating method which yields a greater precoat effect at substantially lesser costs and which is more economical in its handling than precoats and precoating methods heretofore used.

Other objects of this invention are the application of a precoat at substantially higher coating speeds than possible heretofore and which at the same time yields prints of improved evenness in full and intermediate tones.

Precoating constitutes a pretreatment of reproduction paper base stock with inert materials, prior to the application of a light sensitive coating, with the purpose of achieving an improved print quality. Precoating was first introduced and wasso named in United States Letters Patent No. 2,433,515, granted on Dec. 20, 194-7, to Edward Jahoda, assigned to H. P. Andrews Paper Company (now Reissue Patent No. Re. 23,510, granted June 10, 1952).

Before the advent of precoating, the efforts to improve print quality in reproduction papers were restricted either to the paper base or to the composition and application of the sensitizing solution. Patent No. 2,433,515 provided :an inert coating of colloidal silica, which was applied to regular diazotype or blueprint base paper and then dried, forming a precoat layer on to which the sensitizing layer is superimposed. The precoat layer has the effect of increasing the density and thereby the contrast of the final print as compared to the print density that could be obtained by the same sensitizing solution on the same base stock but without the intermediate precoat layer.

A pre-coat of the type disclosed in the above patent is not intended to change and, as a matter of fact, does not change the appearance or the feel of the paper stock and its only specific effect is the increase in density of the print resulting after superimposing a sensitized layer on the precoat and making a print therefrom.

Special purpose reproduction papers have been manufactured in the past on modified base stocks. The use of baryta coated base stock provided a smooth matted print surface. Organic solvent resin coatings yielded glossy prints and water fastness. These modified base materials, however, did not bring about the above defined precoat effect; in other words, they did not contribute to an increase of the print density. On the contrary, they frequently flattened the print densities and decreased the rateof development of the prints. They rendered the coating with the light sensitive layer more diificult, insofar as they required the inclusion of special solvents in the coating solution or the use of specialized coating techniques.

Other types of inert pretreatments of coated materials are known for the purpose of making a hydrophilic surface adhere to a hydrophobic surface or vice versa, or to close the pores of a porous surface in order to facilitate the application of a film forming resin solution. Such coatings are known as substrate or subbing or primer; and they make hydrophobic surfaces hydrophilic or vice ver'sa or prevent penetration of the final top coating. They generally contain large amounts of organic solvents or swelling agents for the base material and accordingly are not applicable to and do not improve print density if applied to the wettable surfaces of the ordinary reproduction base papers.

In considering precoating of the type with which we are concerned, it should be noted that such precoats do not contribute to keeping the sensitizing solution on the surface of the base as sometimes wrongly assumed. Thus, the precoat effect can be obtained on completely porous papers where the sensitizing solution penetrates through the base sheet. Such precoated absorbent papers are for instance useful for chromatographic purposes. However, for reproduction papers, it is generally desired to keep the sensitizing layer as close to the surface as possible and as compatible with good anchorage and uniform and sufficient depth of the image forming ingredients.

Sensitizing layers which penetrate deep into the base stock yield flat looking prints. Sensitizing layers which penetrate too little, staying mostly on top of the base stock, yield prints with the undesirable effect known as bronzing and at times adhere poorly to the base stock. Controlled pentration to a certain well defined depth is necessary to assure optimum brilliancy of the print, color and smoothness of the intermediate tones.

When coating and drying speeds were generally increased in recent years, the precoats formerly used did not allow control of the penetration to its optimum degree. Under such conditions they yield prints with little smoothness and the print colors come off easily when the prints are handled.

For the purpose of controlled pentration, reproduction base papers generally are surface sized which restricts the penetration. Due to the fibrous character of the paper, the sizing and consequently the penetration are not uniform across the sheet surface. Precoating does not effect the penetration resistance of the sized base paper in any way. In other words, it does not improve it nor does it detract from it. Generally speaking, the combination of a sized base paper with a precoat is especially useful.

The precoat eflect is a physical effect depending on the discontinuity of the precoat layer. The discontinuous precoat layer is an artificial surface, inert to but more receptive to the sensitizing solutions and with a higher affinity for the final print dyes than any natural coating surface. Silica was the first, and is still, a highly effective precoat chemical. Yet the precoat effect is not dependent on its chemical nature. Colloidal silica can be applied to most fibrous surfaces essentially without binder and will adhere thereto due to its own nature. But the precoat effect can be obtained by materials of larger than colloidal size if their adherence to the base and their coherence is assured by suitable binders.

Many attempts have been made to achieve the precoat effect by the inclusion of pigments or resins in the sensitizing solution. Such solutions came to be known as pseudo precoat or one pot solutions alluding to the inclusion of precoat chemicals such as silica or aluminum oxide (A1 0 referred to as alumina) or small angular grain starch in one ot with the sensitizing solution. While the pseudo precoats lead to lasting improvements over the prior clean sensitizing solutions, they often resulted in prints showing streaky coating patterns. They can, however, themselves be applied to precoated paper to augment the precoat effects. Thus, precoating stimulated improvements of the sensitizing solution, but it cannot be replaced by the same.

The precoat process is now a generally accepted and established part of the art. While precoats containing colloidal silica, such as disclosed in the above referred to Patent No. 2,433,515 have generally been the most satisfactory for a long time, it has also been proposed to form the precoats from various other materials, such as non-colloidal silica and a suitable binder or from finely divided pure aluminum oxide with a suitable binder. In addition, various synthetic resins, combinations of silica and synthetic resins, metal silicates, titanium dioxide and calcium fluoride have been suggested as precoats.

More recently pigmented precoats have been used very successfully, such as complexes of silica, barium sulfate, and starch particles. These pigment complexes excel by their synergistic effect over the precoat effect of the single components of the prior art.

The pigment complex precoats offer a great advantage over the earlier precoats insofar as they can be applied in layers of a substantially higher coating weight of about 1.2 to 2.5 grams per square yard (dry substance). Besides showing a pronounced precoat effect per se, they can be used to upgrade the base paper by almost completely covering the fibrous character of the paper surface, increasing the brightness of the background and yielding prints with fine intermediate tones.

Notwithstanding the excellent qualities of the pigment precoats based on the silica, barium sulfate, starch complex, certain difficulties turned up in their application due to either the starch or the barium sulfate component. The adherence of the pigment complex to the base paper and the coherence of the pigment particles was sometimes deficient in spite of the use of binders in high concentrations and, at times, sedimentation took place.

It is the prime object of the present invention to overcome the difficulties heretofore encountered in the precoating of reproduction paper and to provide a new precoat and a new precoating method which not only improves adherence of the precoat to the base sheet and coherence of the precoat layer, but also increases the precoat effect, tends less to sedimentation, lowers the cost of precoating, upgrades the base paper, spreads more evenly on the paper and can, therefore, more easily and more economically be applied on high speed coating equipment of paper mills or on conventional diazotype coating machines.

Our invention contemplates compounding a pigment complex of silica particles with a particle size of below 3.5 microns and of aluminum hydroxide (also called hydrated alumina) particles with particle sizes below 1.5 microns, adding thereto suitable binders and applying it as a precoat to a base sheet.

In the accompanying drawing we have indicated a cross-sectional view of reproduction paper embodying our invention. It will be seen that the reproduction material consists of a base sheet or layer of natural paper, cloth or impregnated or plastic coated paper. Our improved precoat is applied to the surface of the base sheet and the light sensitive layer is, in turn, applied thereto. The precoat layer as shown is discontinuous although as indicated the particles agglomerate and where to an extent sufficient to prevent or minimize rub-off when handled. When the light sensitive layer is applied to the precoat layer it not only fills the interstices between the particles but also there is a certain amount of interpenetration between the precoat layer and the light sensitive layer as indicated by the cross sectional lines in the drawing and by the dotted lines which show the depth of interpenetration.

Our improved precoat consists of a uniform composition or complex of particles of silica of a particle size below 3.5 microns and of particles of aluminum hydroxide of a particle size below 1.5 microns and of suitable binders.

The silica should be preferably partly of a particle size below 0.1 micron and partly of particle sizes between 0.1 micron and 3.5 microns. The silica may be in dry form of a fluffy powder or it may be applied from aqueous dispersions. If our precoat is used in connection with diazotype reproduction paper, the silica must meet the rigid purity requirements of this process and must be substantially free from iron.

The aluminum hydroxide, sometimes referred to as hydrated alumina or hydrated aluminum oxide is generally represented by the chemical formulations: Al(OH) or Al O .3H O. It contains about 35% chemically bound water which remains in the molecule during and after the application. The expression aluminum hydroxide as used in this patent application shall embrace materials of the above indicated type having approximately 35% chemically bound water. In this connection, we prefer to employ synthetically produced aluminum hydroxide. For best results the particle size of the aluminum hydroxide particles should be below 1.5 microns.

The precoat pigment complex is preferably applied to the base material in the form of an aqueous dispersion. Suitable binders should be included in the aqueous pigment dispersion for a better coherence and sufficient adherence to the base material. Any of the known binders used for this purpose can be employed, such as non ionic or anionic or cationic aqueous resin dispersions of polyvinyl acetate, polyvinyl alcohol, gelatin, gum arabic, polyvinylpyrrolidone, solubilized casein, or the like.

The precoat pigment complex coating will form a layer of discontinuous particles embedded in a continuous system. The continuous nhase of the system is the binder.

The proportions of the silica and the aluminum hydroxide to each other may be varied depending upon the particular effects desired. The small silica particles, when used alone, produce sparkle, brilliance and color. The larger sized particles produce uniformity. Aluminum hydroxide, when used alone, produces smoothness in the print half tones but does not enhance the color value of the print color. The larger sized silicas between 0.1 micron and 3.5 microns, when used alone, do not readily bind to the base material and the rather poor flow properties of their aqueous dispersions result in streaky coating patterns.

Surprisingly, combinations or complexes of silica particles and aluminum hydroxide particles produce a synergistic precoat effect. Line color and density and color value of prints with such precoat material excel over prints made from diazo reproduction materials precoated with either only one of the silicas or aluminum hydroxide. Furthermore, the flow characteristics of silicaaluminum hydroxide precoat pigment are superior and yield highly even coatings free of streaks and patterns. In addition, they can be easily bound to the base sheet and cohere well if used with the above mentioned conventional binders.

While we do not intend to limit ourselves to a particular theory regarding the synergistic effect of the complex, it appears to us that the composition of above defined particles of silica and aluminum hydroxide disperse on each others surfaces and form complexes held together by forces of electrical or Van der Waal nature, which greatly modify the surface characteristics, being inert to the sensitizing solutions and having an increased aflinity for the azo dye of the print.

Unexpectedly, we have found in the application of light sensitive diazotype solutions onto our improved precoat, that the development promoting agents can be omitted or considerably decreased in concentration, without lowering the required development rate. In addition, on non precoated sheets, or sheets with conventional precoats, diazotype solutions with coupler to diazo ratio of 3 to 1 or even up to to 1 are required to obtain adequate print contrast. We have found that we obtain very satisfactory print contrast with a much lower coupler to diazo r-atio such as 2 to 1. We have also found that with our improved precoats, the concentration of the stabilizing agents, like citric acid in diazotype coatings can be reduced without impairing the self life of the sensitized paper.

. The elimination or decrease of developing agents, the lesser coupler to diazo ratio, and the lower concentration of stabilizers (citric acid) are three important simplifications of the make-up of the diazotype solution which amount to substantial savings in the total cost of coating materials. As a matter of fact, the savings are greater than the cost of our improved precoat.

We have found that we can obtain the synergistic precoat effect within the following ratios of the components: above defined particles of silica 5% to 60% and above defined particles of aluminum hydroxide 95% to 40% of the combined pigments.

Sufiicient binder should be added for adequate coherence of the pigment particles and adherence to the base sheet. We have found that we can achieve satisfactory results if the binder added is between 8% and 30% by weight of the combined pigments.

It can be easily seen from the composition of this precoat that it offers, besides the improved precoat effect, an economic advantage over previous precoats, aluminum hydroxide being a much less expensive precoat component than earlier employed precoat components like blanc fixe, rice starch, polystyrene and others.

I The much lower tendency of this improved precoat to form a sediment during the processing operation constitutes another cost saving factor.

- In the preferred embodiment of our invention, we mix approximately 1 part of silica with about 3 parts of aluminum hydroxide and add binder within the indicated limits to cause the pigment particles to cohere and to adhere to the base sheet while maintaining a discontinuous coating layer of separate and discrete particle aggregates.

While we prefer to apply the components in form of a genuine precoat, it is also possible to add them in the sensitizing solution, as a pseudo precoat.

When using this invention, in the form of a genuine precoat, it is possible to improve the line density still further by adding excess silica to the sensitizing solution. Silica may be contained both in the precoat and in the sensitizing solution.

In making our improved precoat, a slurry is first prepared by mixing the silica and aluminum hydroxide in the indicated proportions with water. The different types of particles may be combined in dry form and then added together to the water or each may be dispersed separately in water. The resin binder is preferably prepared in a separate aqueous dispersion and this dispersion is then added to the pigments. The proportion of water to solid material of the slurry may be varied, but we have found that best results are obtained if the solid materials make up from between 10% to 40% of the mix. After the various materials have been thoroughly mixed to yield a uniform distribution of all the components, the precoat is then ready to be used. It may be applied to the base sheet either at the paper mill, using conventional high speed coaters, such as the Ross off-machine airknife coater, or it may be applied at the sensitizing plant using conventional blueprint or reproduction paper coating machines.

The thickness or weight of the precoat may be varied. By using a relatively heavy precoat layer, we can upgrade the base stock and increase its subs-t ance weight. This may be obtained by applying three grams of precoat material (dry weight) to each square yard of base stock. However, very satisfactory precoat effects can be obtained by using a substantially lighter precoat layer; we have obtained very satisfactory results by using as little as one gram precoat (dry weight) per square yard of base stock.

Depending on the nature of the hinder, the precoats of our invention may exhibit a slight curing effect and may improve on standing for several days. The precoated layers may have the sensitizing coating applied immediately after precoating or they may be stored or shipped to another point to have the sensitizing coating applied at a later date. Any suitable light sensitive layer may be applied to the precoated sheets, such as a blueprint coat-- mg layer, a diazo solution, silver emulsion, or a Van Dyke solution. The precoated reproduction paper thus formed is of the general type shown in the accompanying drawing. When the precoat is applied to the base sheet at the paper mill by a high speed coater, it is usually desirable to increase the viscosity of the precoat preparation. This can be accomplished by using a relatively higher proportion of binder.

We have found that a precoat layer embodying our invention is not in the form of a rubbery soft mass or coherent film. The particles agglomerate to some extent, but remain nevertheless in a discontinuous, finely divided form as shown in the drawing; the precoat layer, if not supported, is brittle. A layer of this type provides the desirable physical conditions for receiving the light sensitive layer and for enhancing the sparkle, brilliance and color of the dye in the finished print and for improving the smoothness in full and half tones and increasing the apparent whiteness of the sheet.

In the following examples illustrating specific embodiments of our invention, the substance weight means the weight in pounds of five hundred sheets, 17" x 22 in size:

Example I A precoat composition is prepared by mixing the following materials:

Water cc 1,000 Silica (having a particle size below 3.5 microns) 35 Aluminum hydroxide (of a particle size below 1.0

micron) g Dispersion of polyvinyl acetate, non ionic (40% solids) g 35 Gelatin g 10 At a paper mill, the precoat composition thus prepared was applied by an off-machine airknife coater made by Ross to a sulfite diazo base paper of 20 /2 lb. substance weight at a coating speed of 600 f.p.m. and dried by hot air at a temperature of 250 F.

After precoating, the substance weight of the paper was 22 lbs. A satisfactory reproduction paper may be produced, either immediately or at a later date, by sensitizing the precoated paper with a conventional ammonia blueline solution, such as:

Water cc 1,000 Citric acid g 20 Thiourea g 30 Zinc chloride g 30 2,3-dioxy-6 sulfonaphthalene sodium salt g 40 Para diazo diethylaniline zinc chloride double salt g 20 Diethylene glycol cc l5 Saponine g 1 After sensitizing, 250 F.

The prints of this paper showed considerably deeper color and line density, more brilliance and better smoothness in the intermediate tones than prints made from the paper was dried by hot air of paper precoated with a disperion of silica or hydrated alumina alone. The visual whiteness of the background is increased considerably over that of the sensitized nonprecoated base paper.

Due to the increase in substance weight, the material costs of the precoated paper are lower than for regular 22 lb. diazo paper.

Example 2 A precoat composition is prepared by mixing the following materials: Water cc 1,000 Silica (of a particle size below .02 micron) g 7 Silica (of a particle size between 0.1 micron and .15 micron) 28 Aluminum hydroxide (of a particle size below 1.0

micron) 105 Dispersion of polyvinyl acetate (40% solids) g 25 Casein g 14 Borax g 1.1 Ammonia, concentrated g 2.2

Water cc 1,000 Citric acid g 30 1,3,6-naphthalene trisulfonic acid g 25 P-salt g 40 Thiourea 40 p-Diazo-N-oxyethyl-N-ethyl aniline zinc chloride double salt g 20 Diethylene glycol cc 20 Prints made from this paper excelled by their sparkle, line density and smoothness in the intermediate tones over prints made with paper precoated with either the silicas or aluminum hydroxide alone or precoated with synthetic resins precoats, for instance, polystyrene or with pigmented precoats of silica, blanc fixe and starch.

Example 3 A precoat composition is prepared by mixing the following materials:

Water cc 1,000 Silica (of a particle size between .02 and .15

micron) g 50 Aluminum hydroxide (of a particle size of 0.8

micron in average) g 90 Dispersion of styrene-butadieue-copolymer (40% solids) g 25 Polyvinyl alcohol, low viscosity g 14 On a reproduction paper coating machine equipped with two front and one back coating stations, each consisting of an applicator roller and an airknife and a convection hot air dryed after each coating station, the precoat thus prepared was applied to a 20 lb. sulfite base paper, at a coating speed of 150 f.p.m. and dryed by hot air at a temperature of 250 F.

The substance weight of the paper increased to 22 lbs. The precoated paper thus formed was ready to be immediately coated with a light sensitive layer or if preferred, it was suitable for storing or shipping to be coated at a later date. A satisfactory reproduction paper was produced by sensitizing the precoated sheet with the following conventional semi-moist process diazo solution:

8 Water cc 1,000 Ammonium oxalate g 15 Oxalic acid g 8 Aluminum sulfate g 8 Thiourea g 2 p-Diazo benzyl-ethyl aniline zinc chloride double salt g 20 Prints made from this paper and developed with a. conventional alkaline phloroglucinol-resorcinol developer showed deeper line color of higher density, more contrast, better coverage, more smoothness in half tones than prints made from paper which was precoated either with a dispersion of silica or of aluminum hydroxide alone, or precoated With a dispersion of starch with or without silica and blanc fixe. The visual whiteness of the background was increased considerably over that of a sensitized sheet made by use of the mentioned conventional precoat.

Example 4 A precoat composition is prepared by mixing the following materials in:

Water cc 1,000 Silica (with an average particle size of 0.1

micron) g 60 Aluminum hydroxide (of a particle size of below 1.0 micron) g 40 Gum arabic g 10 Gelatin g 5 On a blueprint paper coating machine of conventional design, equipped with a precoat station, consisting of an applicator roller, and an airknife and a sensitizing station, consisting of two applicator rollers, and an airknife and a hot air convection drier pass after each coating operation, the precoat thus prepared was applied to a 50% rag content paper, 30 /2 1b. substance weight, at a coating speed of f.p.m.

After drying the precoated paper, the substance weight was 22 lbs. The precoated paper thus formed was ready to be immediately coated with a light sensitive layer or, if preferred, it was suitable for storing or shipping to be coated at a later date. A satisfactory reproduction paper was produced by sensitizing the precoated sheet with the following blueprint coating solution:

Water cc 1,000 Urea g 45 Washaid g 24 Potassium ferrocyanide g 60 Potassium oxalate g 45 Ferric ammonium oxalate g Potassium ferricyanide g 30 Prussaid g 25 Potassum persulfate g 2.5 Saponine g 12 Prints made from this paper, after drying, showed more sparkle, deeper blue color, better contrast, more smoothness in full and intermediate tones than paper precoated with either the indicated concentration of silica or aluminum hydroxide alone. The prints excelled by their good washout properties.

Example 5 A precoat composition prepared by mixing the following materials:

Water cc 1,000 Silica (of an average particle size below 0.5

micron) g 7 On a reproduction paper coating machine, such as the one cited in Example 2, and under identical conditions, an identical base paper was precoated with above precoat preparation and sensitized with the following blueline ammonia process diazo solution:

Water cc 1,000 Citric acid g 20 Thiourea g 30 Zinc chloride g 30 2,3-dioxy-6-sulfonaphthalene, sodium salt g 40 p-Diazo diethylaniline zinc chloride double salt g 20 Diethylene glycol cc 15 Saponine g 1 The prints of this paper showed considerably deeper color and line density, more brilliance and better smoothness in the intermediate tones than prints made from paper precoated with a dispersion of silica or aluminum hydroxide or polystyrene alone or a pigment mixture of silica, blanc fixe with or without rice starch. The visual whiteness of the background was increased considerably over the sensitized non-precoated base paper.

It is understood that in each of the foregoing examples, the proportions of the ingredients may be varied within the indicated limits and that different binders may be substituted. When it is not desired to increase the substance Weight of the base paper stock, a lighter precoat layer may be applied.

From the foregoing description and examples of our precoated reproduction paper and method of making the same, it will be seen that we obtain a greater precoat elfect than precoats or precoating methods heretofore employed; that the precoat can be readily applied to the base sheet either at the prevailing speeds or at the much faster speeds used by paper mills and that substantial upgrading of the base sheet stock can be obtained. Finally, important savings can be achieved due to the lower materials cost, aluminum hydroxide being a particularly low cost pigment.

Modifications may be made in the illustrated and described embodiments of our invention without departing from the invention as set forth in the accompanying claims.

We claim:

1. An improved precoated sheet for supporting a light sensitive reproduction layer comprising a backing sheet having applied thereto precoat particles comprising an intimate uniform mixture of silica particles having a particle size no greater than approximately 3.5 microns and aluminum hydroxide particles having a particle size no greater than approximately 1.5 microns in the proportion of approximately 5% to by weight of silica particles and to 40% by weight of alumnium hydroxide particles and a binder in the proportion of between approximately 8% and 30% by weight of combined silica and aluminum hydroxide particles, said precoat particles being in discontinuous, finely-divided form in said binder.

2. An improved precoated sheet for supporting a light sensitive reproduction layer as set forth in claim 1 in which a portion of the silica particles have a particle size of less than 0.1 micron and the remainder of the silica particles have a particle size of between 0.1 and 3.5 microns.

3. An improved diazo reproduction sheet comprising: a backing sheet having applied thereto a precoat layer; said precoat layer comprising an intimate uniform mixture of silica particles having a particle size no greater than approximately 3.5 microns and aluminum hydroxide particles having a particle size of no greater than approximately 1.5 microns and the mixture of silica particles and aluminum hydroxide particles is in the proportion of approximately 5% to 60% by weight of silica particles and 95% to 40% by weight of aluminum hydroxide particles; said precoat layer being in discontinuous, finely-divided form and said reproduction sheet having applied thereto a light sensitive reproduction layer.

4. An improved diazo reproduction sheet as set forth in claim 3 in which a portion of the silica particles have a particle size of less than 0.1 micron and the remainder of the silica particles have a particle size of between 0.1 and 3.5 microns.

5. An improved diazo reproduction sheet as set forth in claim 3 in which the precoat layer includes a binder in the proportion of between approximately 8% and 30% by weight of the combined silica and aluminum hydroxide particles.

References Cited UNITED STATES PATENTS Re.23,510 6/1952 Jahoda 117 34 2,433,515 12/1947 Jahoda "11734 2,662,013 12/1953 Sulich 61; al. 9675 2,734,835 2/1956 Florio etal. 117-169 WILLIAM D. MARTIN, Primary Examiner.

H. W. MYLIUS, M. LUSIGNAN,

Assistant Examiners. 

3. AN IMPROVED DIAZO REPRODUCTION SHEET COMPRISING: A BACK SHEET HAVING APPLIED THERETO A PRECOAT LAYER; SAID PRECOAT LAYER COMPRISING AN INTIMATE UNIFORM MIXTURE OF SILICA PARTICLES HAVING A PARTICLE SIZE NO GREATER THAN APPROXIMATELY 3.5 MICRONS AND ALUMINUM HYDROXIDE PARTICLES HAVING A PARTICLE SIZE OF NO GREATER THAN APPROXIMATELY 1.5 MICRONS AND THE MIXTURE OF SILICA PAR- 